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Streptococcus suis is an important cause of different pathologies in pigs and humans, most importantly fibrinosuppurative meningitis. Tissue infected with this pathogen is substantially infiltrated with neutrophils, but the function of neutrophil extracellular traps (NETs) -a more recently discovered antimicrobial strategy of neutrophils -in host defence against Strep. suis has not been investigated. The objective of this work was to investigate the interaction of Strep. suis with NETs in vitro. Strep. suis induced NET formation in porcine neutrophils and was entrapped but not killed by those NETs. As the amount of NETs decreased over time, we hypothesized that a known extracellular DNase of Strep. suis degrades NETs. Though this nuclease was originally designated Strep. suis-secreted nuclease A (SsnA), this work demonstrated surface association in accordance with an LPXTG cell wall anchor motif and partial release into the supernatant. Confirming our hypothesis, an isogenic ssnA mutant was significantly attenuated in NET degradation and in protection against the antimicrobial activity of NETs as determined in assays with phorbol myristate acetate (PMA)-stimulated human neutrophils. Though assays with PMAstimulated porcine neutrophils suggested that SsnA also degrades porcine NETs, phenotypic differences between wt and the isogenic ssnA mutant were less distinct. As SsnA expression was crucial for neither growth in vitro nor for survival in porcine or human blood, the results indicated that SsnA is the first specific NET evasion factor to be identified in Strep. suis.

Streptococcus suis serotype 2 is a porcine and human pathogen with adhesive and invasive properties. In other streptococci, large surface-associated proteins (>100 kDa) of the MSCRAMM family (microbial surface components recognizing adhesive matrix molecules) are key players in interactions with host tissue. In this study, we identified a novel opacity factor of S. suis (OFS) with structural homology to members of the MSCRAMM family. The N-terminal region of OFS is homologous to the respective regions of fibronectinbinding protein A (FnBA) of Streptococcus dysgalactiae and the serum opacity factor (SOF) of Streptococcus pyogenes. Similar to these two proteins, the N-terminal domain of OFS opacified horse serum. Serum opacification activity was detectable in sodium dodecyl sulfate extracts of wild-type S. suis but not in extracts of isogenic ofs knockout mutants. Heterologous expression of OFS in Lactococcus lactis demonstrated that a high level of expression of OFS is sufficient to provide surface-associated serum opacification activity. Furthermore, serum opacification could be inhibited by an antiserum against recombinant OFS. The C-terminal repetitive sequence elements of OFS differed significantly from the respective repeat regions of FnBA and SOF as well as from the consensus sequence of the fibronectin-binding repeats of MSCRAMMs. Accordingly, fibronectin binding was not detectable in recombinant OFS. To investigate the putative function of OFS in the pathogenesis of invasive S. suis diseases, piglets were experimentally infected with an isogenic mutant strain in which the ofs gene had been knocked out by an in-frame deletion. The mutant was severely attenuated in virulence but not in colonization, demonstrating that OFS represents a novel virulence determinant of S. suis.Streptococcus suis serotype 2 is an important invasive porcine pathogen worldwide. It also causes meningitis and other diseases in humans. Processing of pork is considered to be a major risk factor of this zoonosis (3). In pigs, septicemia, meningitis, polyarthritis, polyserositis, and pneumonia are common clinical manifestations. These diseases have been reproduced experimentally by infections with S. suis serotype 2. In addition to epidemiological data, experimental infections demonstrated that wild-type serotype 2 strains, which express a 136-kDa muramidase-released protein (MRP) and an 80-kDa extracellular factor (EF), are highly virulent, in contrast to MRP Ϫ EF Ϫ serotype 2 strains from Europe (32, 35). Smith et al. (27) previously showed that the capsule of S. suis protects against phagocytosis. Other factors such as suilysin and the fibronectin-and fibrinogen-binding protein of S. suis (FBPS) may contribute to the pathogenesis of S. suis (1, 9, 19), but so far, the capsule is the only major virulence factor to be identified (27).In other streptococci such as Streptococcus pyogenes and Streptococcus dysgalactiae, a number of large surface-associated proteins, including SfbI, serum opacity factor (SOF), and fibronectin-binding prot...

Streptococcus suis causes numerous diseases in pigs, most importantly, meningitis, arthritis, septicemia, and bronchopneumonia. One of the major problems in modern swine production is the lack of a vaccine protecting against more than one S. suis serotype. The objective of this study was to determine the protective efficacy of a serotype 2 murein-associated protein (MAP) fraction subunit vaccine in comparison to that of a bacterin against experimental challenge with serotype 2 (containing muramidase-released protein [MRP], extracellular factor, and suilysin [SLY]) and serotype 9 (containing MRP variant MRP* and SLY) strains. MAP was shown to include different surface-associated proteins, such as the MRP and surface antigen one (SAO) expressed by both pathotypes used for challenge. The results of this study demonstrated that the serotype 2 bacterin induced protective immunity against homologous challenge. In contrast, the protective efficacy of the MAP subunit vaccine was low, though MAP immunization resulted in high serum immunoglobulin G2 titers against MRP and SAO. Importantly, immunization with bacterin but not with MAP induced opsonizing antibody titers against the serotype 2 strain, and these antibody titers were found to correlate with protection. However, after absorption with a nonencapsulated isogenic mutant, the sera from bacterin-immunized piglets failed to facilitate neutrophil killing, indicating that antibodies directed against capsule may not have been essential for opsonophagocytosis. Furthermore, induction of opsonizing antibodies against serotype 9 was not detectable in the group receiving bacterin or in the group receiving the MAP vaccine. In agreement, protection against the heterologous serotype 9 strain was low in both groups. Thus, identification of an antigen protecting against these two important S. suis pathotypes remains an important goal of future studies.Streptococcus suis ranks among the five most important health challenges of pigs worldwide (11,12). It is associated with numerous diseases, such as meningitis, arthritis, serositis, and bronchopneumonia. S. suis isolates from diseased animals express a polysaccharide capsule which confers resistance to phagocytosis, as demonstrated for serotype 2 strains (21). Strains of various serotypes have been isolated from affected tissues. In Europe, serotype 2 and 9 strains are the most prevalent types isolated from infections. The 136-kDa muramidasereleased protein (MRP) and the 110-kDa extracellular factor (EF) are virulence-associated factors expressed only by virulent serotype 1 and 2 strains (22, 23). The majority of invasive serotype 9 isolates express a larger variant of MRP, termed MRP*, which shares high homology with the 136-kDa MRP protein of serotype 2 strains (20, 23).